The present invention relates to a system and a method for improving conversion efficiency of a lean NOx catalyst in a diesel or lean burn gasoline engine, and, more particularly, to improving conversion efficiency by controlling delivery of a NOx reductant.
Internal combustion engines commonly rely on exhaust aftertreatment devices to convert regulated components: carbon monoxide, hydrocarbons, and nitrogen oxides (NOx), into carbon dioxide, water, nitrogen, and oxygen. Exhaust catalysts have been extensively developed to obtain high conversion efficiencies on stoichiometric exhaust gases. Stoichiometric conditions are achieved when the fuel and oxidizer supplied to the engine is in a proportion which, if reaction of the fuel were complete, produce carbon dioxide, water, and nitrogen. It is known to those skilled in the art, though, that higher fuel efficiency is obtained from engines operating at air-fuel ratios lean of stoichiometric, that is, with an excess of air. These lean burning engines may be diesel engines, stratified-charge gasoline engines in which the fuel and air are only partially mixed, and homogeneous-charge, lean-burn gasoline engines in which the fuel and air are mostly premixed prior to combustion. Because of the desire for high fuel efficiency, lean burning engines are in production and continue to be developed. It is known to those skilled in the art to use a NOx catalyst and continuously supply reductant to the catalyst to convert NOx while operating lean.
The inventors of the present invention have recognized that if reductant is supplied when the catalyst is at high temperatures, the subsequent NOx conversion efficiency of the catalyst is higher than heretofore possible in the 140-250xc2x0 C. temperature range.
The inventors of the present invention have recognized that substantially higher NOx conversion efficiencies of a lean NOx catalyst can be achieved by supplying reductant when the temperature in the lean NOx catalyst is greater than about 300xc2x0 C. The inventors recognized that a lean NOx catalyst may periodically achieve the desired temperature range and reductant may be supplied to the lean NOx catalyst in response.
Disadvantages of prior approaches are overcome by a method for controlling reductant addition to exhaust gases of an internal combustion engine. The reductant and exhaust gases flow into a catalyst coupled to the engine. An indication that temperature of the catalyst is higher than a predetermined temperature is provided. In response to the indication, reductant is added into the exhaust gases. An estimate of a stored quantity of reductant within the catalyst is provided. The reductant addition step is substantially discontinued when the stored quantity exceeds a predetermined quantity.
A primary advantage of the present invention is that a lean NOx catalyst processing lean exhaust gases operates with substantially higher conversion efficiency in a lower temperature range than heretofore possible. Another advantage of the present invention is that after storing reductant under prescribed conditions, reductant delivery can be lessened or discontinued. Furthermore, significantly less reductant is supplied to the catalyst than prior art methods.
Yet another advantage of the invention herein, over prior art, is that because less reductant is supplied to the catalyst, less reductant slips through the catalystinto the tailpipe.
The above advantages and other advantages, objects, and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings.